1. Field of the Invention
The invention relates to a multilayer information recording medium intended for an optical disk, and a method and apparatus for recording/reproducing the multilayer information recording medium.
2. Description of the Prior Art
There have been known a variety of principles for recording information on a thin film (recording film). Of these, ones that utilize changes in atomic arrangements resulting from laser light irradiation, such as a phase change (also referred to as phase transition) and photo darkening, involve little deformation of the thin film. This means an advantage that two disk members can be directly bonded together to obtain an information recording medium of double-sided disk structure, or a multilayer information recording medium having a plurality of information planes.
A document 1 (Japanese Patent Laid-Open Publication No. Hei 5-101398) describes a method of reproducing information from a multilayer information recording medium, in which a reproducing head is moved to a constant distance from the information plane to reproduce. In this method, a deviation in the amount of movement of the head can preclude precise focusing on the information plane, making the accurate reproduction of information difficult.
For multilayer ROM disks, as described in a document 2 (DVD Specifications for Read-Only Disc), there has been used a method of recording layer numbers, or address information, to pits formed on the respective information planes, and reproducing the same. In this method, each layer is put under focus and then tracked to reproduce the address information, which is subjected to signal processing before the layer number can be determined. As a result, it takes much time before information is reproduced with accuracy.
As employed in this specification document, an optical disk refers to a disk on which information reproducible through light irradiation is recorded, and/or an apparatus for reproducing information through light irradiation.
In conventional multilayer information recording media, it is hard to accurately determine an information plane for reproduction. Moreover, in the method intended for multilayer ROM, in which each layer is put under focus and then tracked to reproduce address information and the address information is subjected to signal processing before the layer number is determined, it takes much time before information is reproduced with accuracy.